1. Field of the Invention
The present invention relates, in general, to radiators or heat exchangers and, more particularly, to vehicle radiators.
2. Description of the Art
Heat exchangers are used in various applications to remove waste heat from industrial processes. In the case of a vehicle, a radiator is employed to remove heat or combustion from the engine. The vehicle radiator includes a core which is connected in fluid communication with fluid passages through the engine block to circulate coolant through the block. The coolant picks up heat from the engine block and radiates the heat through radiator fins as it circulates through the radiator. An engine driven fan is provided along one side of the radiator to provide a cooling air flow onto the fins to increase the heat exchange rate, particularly when the vehicle is not in motion or is operating at low speed insufficient to generate a high speed air flow onto the radiator.
While vehicle radiators with engine driven fans have been effectively used for many years in millions of vehicles, a problem always exists when a radiator loses efficiency, coolant or the fan belt breaks. If not immediately detected, the loss of cooling capacity can result in serious if not fatal damage to the engine. Even if detected, a loss of cooling efficiency results in overheating of the engine coolant thereby requiring the engine to be shut off and the vehicle rendered immobile for an extended period of time until the coolant temperature decreases.
Thus, it would be desirable to provide an auxiliary or emergency heat exchange apparatus which removes waste heat from a two phase fluid circulating in a heat generating apparatus to provide adequate cooling upon deactivation of the main heat exchanger or radiator. It would also be desirable to provide such a heat exchange apparatus which can be easily mounted in an existing cooling system, such as a vehicle cooling system, without requiring major modification to the cooling system. It would also be desirable to provide an auxiliary or back-up heat exchange apparatus which utilizes condensation phenomenon.
It is known in the film-wise condensation of vapor that latent heat of condensation passes through a film of liquid on its way to the condensation surface. The predominant mode of heat transfer through the film is conduction. Since most liquids have a low thermal conductivity, the condensate film provides a substantial resistance to heat transfer. If the condensate film is not removed from the condensing surface, it thickens and increases the resistance to heat transfer. In most stand alone industrial condensers, the condensate continually drains away from the cooling surface by gravity.
It is well recognized that centrifugal forces generated in a rotating system may be utilized to replace the gravity force in the condensation process. Condensation may be film-wise when there is a continuous flow of liquid over the cooling surface, or drop-wise when the vapor condenses in droplets and the cooling surface is not completely covered by liquid.
After a condensate film is developed in film-wise condensation, additional condensation will occur at the liquid-vapor interface, and the associated energy transfer must occur by conduction through the condensate film. Drop-wise condensation, on the other hand, always has some surface present as the condensate drop forms and runs off. Drop-wise condensation is, therefore, associated with a higher heat transfer rates of the two types of condensation phenomenon.
Specifically, because of the mechanism of drop-wise condensation, heat transfer coefficients can be about four to twenty times those of film-wise condensation. Additives to promote drop-wise condensation by preventing the condensate from wetting the surface have been used with varying degrees of success, and are effective only for limited periods of time.
Drop-wise condensation is attractive for applications where extremely large heat transfer rates are desired. However, because of its uncertain nature and the conservative approach needed in the design of heat transfer systems, film-wise condensation heat transfer coefficients are predominantly used.